Molecular memory and method for manufacturing molecular memory

ABSTRACT

A molecular memory recording molecular polarization of a single-molecule electret, and the single-molecule electret includes a cluster skeleton 100 having a continuous hole 101 and a plurality of stable ionic sites 102a, 102b and a metal ion M. The molecular polarization is shown in a state in which the metal ion is included in the stable ionic site. The molecular polarization is changed by movement of the metal ion to the other hollow stable ionic site by application of an electric field. The recordkeeping time of the molecular memory in a temperature range of −100° C. to 100° C. based on the ion radius of the metal ion is 3.0×10−2 seconds to 9.1×1022 seconds. Based on the recordkeeping time, the molecular memory is used as any of a volatile memory, a non-volatile memory, and a storage class memory.

TECHNICAL FIELD

The present invention relates to a molecular memory and the method formanufacturing the molecular memory.

BACKGROUND ART

A ferroelectric material has an electric dipole in a crystal, and theelectric dipole is aligned in the crystal even in a state in which thereis no electric field. For the ferroelectric material, the polarizationdirection and level of the electric dipole are controllable byapplication of an electric field. A change in the polarization directionand level by application of the electric field occurs due to movement ofa ferroelectric domain wall. Even when the electric field is cut off,the ferroelectric domain wall does not fully return to an originalstate, and for this reason, P-E hysteresis occurs. Thus, theferroelectric material can record information depending on the directionof the polarization remaining in the state without the electric field.

Meanwhile, there has been a single-molecule electret capable of showingferroelectric properties and behavior with a single molecule. Theferroelectric properties and behavior indicate appearance of the P-Ehysteresis and spontaneous polarization. Unlike the mechanism forcausing hysteresis in a general ferroelectric material, thesingle-molecule electret shows the P-E hysteresis and the spontaneouspolarization based on a slow polarization relaxation phenomenon.

One of molecular structures used for the single-molecule electrets isPreyssler-type polyoxometalate (POM). The Preyssler-type POM is amolecular metal oxide cluster having a ring-shaped structure. ThePreyssler-type POM is represented by [M^(n+): P₅W₃₀O₁₁₀]^((15-n)-). Notethat “P₅W₃₀O₁₁₀” indicates a cluster skeleton and “M^(n+)” indicates ametal ion included in the cluster skeleton. In the cluster skeleton ofthe Preyssler-type POM, two stable ionic sites shifted from the centerare present. An ion taken into the cluster skeleton is stably held inany of the stable ionic sites.

For example, Patent Document 1 discloses that a molecular metal oxidecluster which is stable in holding molecular polarization and stable interms of a structure can be provided.

Patent Document 2 discloses that a multiferroic material including adysprosium ion in one inclusion portion of a cluster skeleton shows P-Ehysteresis and magnetic hysteresis with a single molecule.

Patent Document 3 discloses that a molecular metal oxide cluster showingmolecular polarization functions as a memory when used as asingle-molecule electret layer in a field-effect transistor.

Non-Patent Document 1 discloses such a polyoxometalate molecule that apotassium ion is included in a cluster skeleton.

There are three general types of memories often distributed in themarket. These memories include a volatile memory that loses storedcontents when the power is turned off, such as a DRAM and a SRAM; anon-volatile memory that holds contents for a long period of time evenafter the power has been turned off, such as a ROM and a flash memory;and a storage class memory having intermediate properties between thevolatile memory and the non-volatile memory. The storage class memorythat holds contents for a short period of time even after the power hasbeen turned off can be provided with a higher speed and a largercapacity, and can be produced at low cost. Thus, the storage classmemory has recently attracted attention as a new memory.

CITATION LIST Patent Document

PATENT DOCUMENT 1: Japanese Unexamined Patent Publication No. 2017-95334

PATENT DOCUMENT 2: Japanese Unexamined Patent Publication No. 2018-32813

PATENT DOCUMENT 3: Japanese Patent Application No. 2019-118917

Non-Patent Document

NON-PATENT DOCUMENT 1: Akio Hayashi, Muh. Nur Khoiru Wihadi, Hiromi Ota,Xavier Lopez, Katsuya Ichihashi, Sadafumi Nishihara, Katsuya Inoue, NaoTsunoji, Tsuneji Sano, Masahiro Sadakane, “Preparation of Preyssler-typePhosphotungstate with One Central Potassium Cation and Potassium CationMigration into the Preyssler Molecule to form Di-Potassium-EncapsulatedDerivative,” ACS Omega 2018, 3, 2363 - 2373

SUMMARY OF THE INVENTION Technical Problem

Patent Documents 1 to 3 merely describe that a particular molecularmetal oxide cluster is applicable to some electronic devices. Themolecular metal oxide clusters described in Patent Documents 1 to 3cannot be applied to the volatile memory, the non-volatile memory, andthe storage class memory in a versatile manner, and in some cases, arenot suitable depending on a memory type. For this reason, furtherfindings are necessary for thoughtfully using the molecular metal oxidecluster and applying the molecular metal oxide cluster to variousmemories for promoting the practical use of the molecular metal oxidecluster as a molecular memory.

The present disclosure has been made in view of the above-describedsituation, and is intended to provide a molecular memory configured suchthat a recordkeeping time is controllable across a wide range andconfigured applicable to various memories based on the recordkeepingtime.

Solution to the Problem

For accomplishing the above-described objective, the molecular memory ofthe present embodiment is a molecular memory including a single-moleculeelectret layer having a single-molecule electret showing molecularpolarization and recording the molecular polarization of thesingle-molecule electret in association with a signal of 0 or 1. Thesingle-molecule electret includes a cluster skeleton having a continuoushole and a plurality of stable ionic sites provided apart from eachother in the continuous hole and a metal ion included in any one of thestable ionic sites and formed movable to the other hollow stable ionicsite. The molecular polarization is shown in a state in which the metalion is included in any one of the stable ionic sites. The molecularpolarization is changed by movement of the metal ion to the other hollowstable ionic site by application of an electric field. The recordkeepingtime of the molecular memory in a temperature range of −100° C. to 100°C. based on the ion radius of the metal ion is 3.0×10⁻² seconds to9.1×10²² seconds. Based on the recordkeeping time, the molecular memoryis used as any of a volatile memory, a non-volatile memory, and astorage class memory.

Advantages of the Invention

According to the present embodiment, the molecular memory configuredsuch that the recordkeeping time is controllable across the wide rangeand configured applicable to various memories based on the recordkeepingtime can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing, as viewed in plane, one exampleof a molecular structure of a single-molecule electret according to anembodiment.

FIG. 2 is a schematic diagram showing, as viewed from the side, oneexample of the molecular structure of the single-molecule electretaccording to the embodiment.

FIG. 3 is a schematic diagram of a molecular structure of asingle-molecule electret according Comparative Example 1.

FIG. 4 is a graph showing the electric field dependency of asingle-molecule electret according to Example 1 in association with thepolarization thereof

FIG. 5 is a graph showing the temperature dependency of thesingle-molecule electret according to Example 1 in association with thespontaneous polarization thereof.

FIG. 6 is a graph showing the frequency dependency and temperaturedependency of the single-molecule electret according to Example 1 inassociation with the permittivity thereof.

FIG. 7 is a graph showing the pyroelectric current relaxation time ofthe single-molecule electret according to Example 1.

FIG. 8 is a graph showing the pyroelectric current relaxation time ofthe single-molecule electret according to Example 1.

FIG. 9 is a graph showing the pyroelectric current relaxation time ofthe single-molecule electret according to Example 1.

FIG. 10 is a graph showing the electric field dependency of asingle-molecule electret according to Example 2 in association with thepolarization thereof

FIG. 11 is a graph showing the temperature dependency of thesingle-molecule electret according to Example 2 in association with thespontaneous polarization thereof.

FIG. 12 is a graph showing a correlation between the ion radius of ametal ion and an activation energy when the metal ion moves betweenstable ionic sites.

FIG. 13 is a graph showing a correlation between the ion radius of themetal ion and the recordkeeping time of a molecular memory.

FIG. 14 is a schematic diagram showing the reduction reaction andoxidization reaction of a molecular metal oxide cluster according toExample 1.

FIG. 15 is a graph showing the frequency dependency and temperaturedependency of the molecular metal oxide cluster according to Example 1in association with the permittivity thereof.

FIG. 16 is a graph showing the reversibility of theoxidization-reduction reaction of a molecular metal oxide clusteraccording to Example 2.

FIG. 17 is a flowchart showing the steps of the method of manufacturingthe molecular memory.

FIG. 18 is a block diagram for describing the outline of a storagedevice using a volatile memory and a non-volatile memory according toExample 3.

FIG. 19 is a circuit diagram showing one example of a memory cell of thevolatile memory according to Example 3.

FIG. 20 is a schematic diagram showing, as viewed in plane, a molecularstructure of a single-molecule electret according to Example 4.

FIG. 21 is a schematic diagram showing, as viewed from the side, themolecular structure of the single-molecule electret according to Example4.

FIG. 22 is a graph showing the frequency dependency and temperaturedependency of the single-molecule electret according to Example 4 inassociation with the permittivity thereof.

FIG. 23 is a graph showing the electric field dependency of thesingle-molecule electret according to Example 4 in association with thepolarization thereof

FIG. 24 is a schematic diagram showing, as viewed in plane, anotherexample of the molecular structure of the single-molecule electret.

FIG. 25 is a schematic diagram showing, as viewed from the side, anotherexample of the molecular structure of the single-molecule electret.

FIG. 26 is a schematic diagram showing another example of the molecularstructure of the single-molecule electret.

FIG. 27 is a schematic diagram showing, as viewed in plane, anotherexample of the molecular structure of the single-molecule electret.

FIG. 28 is a schematic diagram showing, as viewed from the side, anotherexample of the molecular structure of the single-molecule electret.

FIG. 29 is a schematic diagram showing, as viewed in plane, anotherexample of the molecular structure of the single-molecule electret.

FIG. 30 is a schematic diagram showing, as viewed from the side, anotherexample of the molecular structure of the single-molecule electret.

FIG. 31 is a schematic diagram showing, as viewed in plane, anotherexample of the molecular structure of the single-molecule electret.

FIG. 32 is a schematic diagram showing, as viewed from the side, anotherexample of the molecular structure of the single-molecule electret.

FIG. 33 is a schematic diagram showing, as viewed in plane, anotherexample of the molecular structure of the single-molecule electret.

FIG. 34 is a schematic diagram showing, as viewed from the side, anotherexample of the molecular structure of the single-molecule electret.

FIG. 35 is a schematic diagram showing, as viewed in plane, anotherexample of the molecular structure of the single-molecule electret.

FIG. 36 is a schematic diagram showing, as viewed from the side, anotherexample of the molecular structure of the single-molecule electret.

FIG. 37 is a schematic diagram showing the reduction reaction of anotherexample of the molecular structure of the single-molecule electret.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present embodiment will be described in detail withreference to the drawings. Description of the preferred embodiment belowis merely an example in nature, and changes can be made as necessarywithout departing from the gist of the present invention.

Molecular Structure of Single-Molecule Electret

FIG. 1 is a schematic diagram showing, as viewed in plane, one exampleof a molecular metal oxide cluster as a molecular structure of asingle-molecule electret according to the present embodiment, and FIG. 2is a schematic diagram from the side. As shown in FIGS. 1 and 2, themolecular metal oxide cluster 10 has a cluster skeleton 100 and a metalion M.

Cluster Skeleton

The cluster skeleton 100 has a plurality of stable ionic sites 102 a,102 b provided apart from each other in a continuous hole 101. In thepresent embodiment, the cluster skeleton 100 is a polyoxometalateskeleton having two stable ionic sites 102 a, 102 b in one continuoushole 101 and represented by a chemical formula of P₅W₃₀O₁₁₀. The clusterskeleton 100 is in the shape of a substantially flat spheroid formedshort in a rotation axis direction and long in a radial direction, hasone continuous hole 101 extending along a rotation axis of thesubstantially flat spheroid, and has the stable ionic sites 102 a, 102 bfor the metal ion M on one and the other opening sides in the continuoushole 101. Of two stable ionic sites 102 a, 102 b, one stable ionic site102 a has the metal ion M, and the other stable ionic site 102 b ishollow. The term “hollow” indicates an empty state in which no ion ormolecule is present and nothing is included.

Metal Ion

The metal ion M is included in any one of the stable ionic sites, and ismovable to the other hollow stable ionic site. In a state in which themetal ion M is included in any one of the stable ionic sites, thesingle-molecule electret shows molecular polarization. In the presentembodiment, the metal ion M included in the cluster skeleton 100 ismovable between two stable ionic sites 102 a, 102 b in the continuoushole 101, and is stable with the metal ion M being included in any ofthe stable ionic sites 102 a, 102 b. When the metal ion M moves from onestable ionic site 102 a to the other hollow stable ionic site 102 b,energy exceeding activation energy is required. In X-raycrystallography, it has been confirmed that disorder of the metal ion Moccurs in an axial direction in the cluster skeleton 100.

In the case of using the cluster skeleton 100, the metal ion M ispreferably one type selected from a group consisting of sodium ions(Na⁺) and lanthanoid ions. For example, the metal ion M is one typeselected from a group consisting of a sodium ion (Na⁺), a gadolinium ion(Gd³⁺), a terbium ion (Tb³⁺), a dysprosium ion (Dy³⁺), a holmium ion(Ho³⁺), an erbium ion (Er³⁺), a thulium ion (Tm³⁺), and an ytterbium ion(Yb³⁺). The molecular metal oxide cluster 10 including any of theabove-described metal ions M shows the molecular polarization.

In the case of using other metal ions such as a calcium ion (Ca²⁺), apraseodymium ion (Pr³⁺), a neodymium ion (Nd³⁺), a samarium ion (Sm³⁺),a europium ion (Eu³⁺), a lutetium ion (Lu³⁺), a cerium ion (Ce⁴⁺, Ce³⁺),an yttrium ion (Y″), a bismuth ion (Bi³⁺), a uranium ion (11⁴⁺), alanthanum ion (La³⁺), and a thorium ion (Th⁴⁺), the molecular metaloxide cluster 10 is also stable, and is common to the above-describedlanthanoid ions in terms of, e.g., an ion radius and electronicproperties. Thus, it can be estimated that such a molecular metal oxidecluster 10 also similarly shows the molecular polarization (see Jorge A.Fernandez, Xavier Lopez, Carles Bo, Coen de Graaf, Evert J. Baerends,Josep M. Poblet, J. Am. Chem. Soc. 2007, 129, 40, 12244 - 12253).

In a case where the cluster skeleton is a polyoxometalate skeleton otherthan the cluster skeleton 100 represented by a chemical formula ofP₅W₃₀O₁₁₀, other metal ions such as a potassium ion (K⁺) may be used asthe metal ion M. Alternatively, as the cluster skeleton, a fullerenewhich can include a metal ion or other inclusion compounds may be alsoapplied. In a case where the cluster skeleton has a plurality of metalions M, these metal ions M are not limited to one type and may be of twoor more types.

In a low temperature range, thermal energy carried by the included metalion M is smaller than the activation energy formed between the stableionic sites 102 a, 102 b in the molecule, and for this reason, the metalion M cannot move from any one 102 a of the stable ionic sites to theother stable ionic site 102 b. Thus, the molecular polarization isfrozen. When an electric field is applied at such a temperature, ionmovement in the molecule is induced so that the molecular polarizationcan be reversed and a molecular polarization direction can be held evenwhen the electric field is cut off. Thus, the single-molecule electretshows, with a single molecule, ferroelectric properties and behavior.

In a high temperature range, the included metal ion M can move, with thethermal energy exceeding the activation energy, from one stable ionicsite 102 a to the other stable ionic site 102 b. Accordingly, themolecular polarization direction becomes uncertain, and a molecularassembly acts like the polarization having vanished. When an electricfield is applied at such a temperature, ion movement in the molecule isinduced so that the molecular polarization can be reversed. However, atthe same time as the electric field being cut off or within a shortperiod of time after the electric field has been cut off, the molecularpolarization direction becomes uncertain.

When an electric field is applied to the molecular metal oxide cluster10, the metal ion M included in the stable ionic site 102 a moves to theother hollow stable ionic site 102 b. The molecular polarization changesdue to movement of the metal ion M between two stable ionic sites 102 a,102 b. Since the cluster skeleton 100 is configured such that thecontinuous hole 101 is provided along the rotation axis, molecularpolarization direction selectivity can be enhanced. That is, selectionof a random direction can be reduced, and uniaxiality can be increased.

COMPARATIVE EXAMPLE 1 Molecular Metal Oxide Cluster 11 Including Waterand Sodium

As a molecular metal oxide cluster reported as an example so far, amolecular metal oxide cluster 11 ([K_(12.5)Na_(1.5)[NaP₅W₃₀O₁₁₀].H₂O])including a sodium ion and a water molecule is synthesized as aprecursor. FIG. 3 shows a schematic diagram of the molecular metal oxidecluster 11 from the side. As shown in FIG. 3, the molecular metal oxidecluster 11 includes the sodium ion (Na⁺) in one stable ionic site 102 a,and includes the water molecule (H₂O) in the other stable ionic site 102b. The water molecule prevents the sodium ion from moving from onestable ionic site 102 a to the other stable ionic site 102 b, and forthis reason, molecular polarization cannot be reversed by application ofan external electric field in the molecular metal oxide cluster 11.Thus, the molecular metal oxide cluster 11 cannot be used as a moleculehaving ferroelectric properties.

COMPARATIVE EXAMPLE 2 Molecular Metal Oxide Cluster Including PotassiumIon

As described in Non-Patent Document 1, in a molecular metal oxidecluster (K₁₃[P₅W₃₀O₁₁₀K]) including a potassium ion (K⁺) as a metal ionM, the potassium ion is positioned substantially at the center betweentwo stable ionic sites. Thus, such a molecular metal oxide cluster showsno molecular polarization. Since the molecular metal oxide clusterincluding potassium as the same alkali metal element as sodium shows nomolecular polarization, no study has been conducted so far aboututilization of a molecular metal oxide cluster including a sodium ion asa single-molecule electret.

EXAMPLE 1 Molecular Metal Oxide Cluster 12 Including Terbium Ion

Patent Document 3 describes that a molecular metal oxide cluster 12including a terbium ion (Tb³⁺) as a metal ion M in a cluster skeleton100 is, as a single-molecule electret, applicable to a non-volatilememory, but there has been no finding about the versatility thereof.

Synthesis of Molecular Metal Oxide Cluster 12

First, the molecular metal oxide cluster 11(K_(12.5)Na_(1.5)[NaP₅W₃₀O₁₁₀]H₂O) including the sodium ion and thewater molecule was synthesized by an already-reported method. Next, 3 mlof H₂O was added to 36.6 mg (0.1 mmol) of Tb(NO₃)₃.6H₂O, and in thismanner, a first solution was prepared. Next, 12 ml of H₂O was added to1.00 g (0.121 mmol) of the molecular metal oxide cluster 11, and in thismanner, a second solution was prepared and was heated to 60° C. Next,the second solution was dropped into the first solution, and in thismanner, a first solution mixture was prepared. Then, the first solutionmixture was held at 145° C. for 24 hours. Next, 4.00 g (53 mmol) of KClwas added to the first solution mixture cooled to a room temperature. Inthe above-described manner, crystal powder was obtained.

By X-ray crystallography, it was confirmed that the resultant crystal isa molecular metal oxide cluster ([TbP₅W₃₀O₁₁₀].nH₂O) including a terbiumion (Tb³⁺). Note that “n” in this crystal was not able to be specified.The resultant crystal was heated at 200° C. to 250° C. as an optimalheating temperature, and in this manner, the water molecule present inthe stable ionic site fully disappears. Accordingly, the molecular metaloxide cluster 12 configured such that the terbium ion (Tb³⁺) is includedin one stable ionic site and the other stable ionic site is hollow wasobtained.

If the heating temperature is lower than 200° C., complete disappearanceof the water molecule from the stable ionic site is difficult. For thisreason, the above-described optimal heating temperature is such atemperature that at least water present in the stable ionic site canfully disappear. The synthesized molecular metal oxide cluster washeated at the optimal heating temperature so that the maximumspontaneous polarization value can be obtained in permittivitymeasurement and a measurement value can be obtained with a higheraccuracy than that in a typical case.

Evaluation of Electric Field Dependency and Temperature Dependency inAssociation with Spontaneous Polarization

For the molecular metal oxide cluster 12 obtained by the above-describedmethod so as to include the terbium ion, a P-E hysteresis was measuredusing a ferroelectric tester (Precision LCII manufactured by RadiantTechnologies). The powder of the molecular metal oxide cluster 12 wascompressed under vacuum, and a pellet disc (an area of 1.33 cm², a filmthickness of 330 μm) was produced as a measurement sample. Note that theP-E hysteresis will be referred to as a D-E hysteresis or a P-Ehysteresis.

FIG. 4 is a P-E hysteresis curve showing the electric field dependencyof the molecular metal oxide cluster 12 according to Example 1 inassociation with the polarization thereof. It was confirmed that afterthe temperature has been increased from 240 K to 325 K, the P-Ehysteresis is shown and the spontaneous polarization increases.Specifically, the P-E hysteresis gradually increased from 240 K, and thespontaneous polarization reached the maximum value at 300 K. Since themolecular metal oxide cluster 12 showed the P-E hysteresis, it was foundthat the terbium ion (T1)³⁺) can move between two stable ionic sites 102a, 102 b by application of an electric field and the molecularpolarization is reversed by movement of the terbium ion between twostable ionic sites 102 a, 102 b.

FIG. 5 is a graph showing the temperature dependency of the molecularmetal oxide cluster 12 according to Example 1 in association with thespontaneous polarization thereof. It can be confirmed that the molecularmetal oxide cluster 12 shows the spontaneous polarization in atemperature range of 270 K to 330 K. Moreover, in a temperature range ofabout 290 K to 310 K, a spontaneous polarization of equal to or greaterthan 1.5 μC/cm² was shown, and the maximum spontaneous polarizationvalue was shown at 300 K.

Evaluation of Frequency Dependency in Association with Permittivity

For the resultant molecular metal oxide cluster 12, the permittivity wasmeasured. First, a pellet (a thickness of 0.169 μm to 0.370 μm, an areaof 1.46 cm² to 133 cm²) was produced from the powder of the molecularmetal oxide cluster 12. Next, gold paste was applied to both surfaces ofthe pellet, and in this manner, an electrode was formed. A wire wasconnected to the electrode, and was further connected to an LCR meter(E4980A manufactured by Agilent). Then, voltage was applied to measurethe frequency dependency and temperature dependency of the molecularmetal oxide cluster 12 in association with the permittivity thereof. Ameasurement frequency was 150 Hz to 2.00 MHz, an applied voltage was2.00 V, and the temperature was 100 K to 400 K.

FIG. 6 is a graph showing the frequency dependency and temperaturedependency of the molecular metal oxide cluster 12 according to Example1 in association with the permittivity thereof. The ferroelectricproperties and behavior of the single-molecule electret are derived froma molecular polarization relaxation phenomenon, and for this reason,there is no clear ferroelectric-induced temperature as in a typicalferroelectric material. Thus, an optional frequency taken as a frequencythat an ion is substantially stopped is set, and a temperature when apolarization relaxation speed reaches such a frequency is taken as ablocking temperature or a hysteresis-induced temperature.

As shown in FIG. 6, in a temperature range of equal to or higher than350 K, a dielectric loss peak depending on the frequency was observed.The temperature at which such a peak is shown shifts to a lowertemperature side as the frequency decreases. This shows that the terbiumion moves between the stable ionic sites 102 a, 102 b and the speed ofmovement between the stable ionic sites 102 a, 102 b decreases as thetemperature decreases.

Evaluation by Measurement of Pyroelectric Current

FIGS. 7 to 9 show graphs the pyroelectric current relaxation time of themolecular metal oxide cluster 12 including the terbium ion. FIG. 7 showsa measurement result at 240 K, and the relaxation time was 8594 seconds.FIG. 8 shows a measurement result at 260 K, and the relaxation time was1593 seconds. FIG. 9 shows a measurement result at 280 K, and therelaxation time was 460 seconds. Note that a pyroelectric current wasmeasured using an electrometer (6517A manufactured by KeithleyInstruments, Inc.).

For the molecular metal oxide cluster 12, different relaxation timeswere measured according to the measurement temperature as describeabove. This suggests that there is no long-range order. This resultshows that when the molecular metal oxide cluster according to thepresent embodiment is applied to various electronic devices, differentresponse speeds are shown under different temperatures. It is assumedthat these properties are also similarly shown by molecular metal oxideclusters including sodium ions and other lanthanoid ions. Since therelaxation time can be controlled by temperature control, the molecularmetal oxide cluster is useful as a non-volatile memory in a certaintemperature range due to a long relaxation time, and is useful as avolatile memory in a certain temperature range due to a short relaxationtime, for example. In a certain temperature range, the molecular metaloxide cluster can be also used as a storage class memory fulfilling anintermediate function between the volatile memory and the non-volatilememory.

EXAMPLE 2 Molecular Metal Oxide Cluster 13 Including Sodium Ion

As shown in FIGS. 1 and 2, a molecular metal oxide cluster 13 includinga sodium ion (Na⁺) as a metal ion M in a cluster skeleton 100 includesthe cluster skeleton 100 having a continuous hole 101 and two stableionic sites 102 a, 102 b provided on one and the other opening end sidesin the continuous hole 101 and the sodium ion (Na⁺) included in onestable ionic site 102 a. Unlike the molecular metal oxide cluster 11according to Comparative Example 1, the molecular metal oxide cluster 13is configured such that the other stable ionic site 102 b include nowater molecule. The other stable ionic site 102 b is hollow.

Synthesis of Molecular Metal Oxide Cluster 13

The molecular metal oxide cluster 13 is synthesized in such a mannerthat the molecular metal oxide cluster 11([K_(12.5)Na_(1.5)[NaP₅W₃₀O₁₁₀].H₂O]) including the sodium ion and thewater molecule is heated at about 300° C. as the optimal heatingtemperature such that the water molecule present in the other stableionic site 102 b fully disappears. Using a single crystal obtained byrecrystallization of the resultant molecular metal oxide cluster 13 withwater, the structure was evaluated by single-crystal X-raycrystallography. As a result, no water molecule was confirmed in thestable ionic sites 102 a, 102 b. Note that in the case of a low heatingtemperature, complete disappearance of the water molecule from thestable ionic site is difficult, and there is a probability that thewater molecule returns into the stable ionic site again. It was foundthat no water molecule returns into the stable ionic site again evenwhen recrystallization with water is performed as long as the watermolecule fully disappears from the stable ionic site by heating at theoptimal temperature.

Evaluation of Electric Field Dependency and Temperature Dependency inAssociation with Spontaneous Polarization

For the resultant molecular metal oxide cluster 13, the P-E hysteresiswas measured using the ferroelectric tester (Precision LCII manufacturedby Radiant Technologies). The powder of the molecular metal oxidecluster 13 was compressed under vacuum, and a pellet disc (an area of1.33 cm², a film thickness of 330 μm) was produced as a measurementsample. FIG. 10 is a P-E hysteresis curve showing the electric fielddependency of the molecular metal oxide cluster 13 according to Example2 in association with the polarization thereof. It was confirmed that asthe temperature increases from 280 K to 420 K, the P-E hysteresisgradually increases and the spontaneous polarization increases.Specifically, there was almost no P-E hysteresis at 280 K, but the P-Ehysteresis was clearly shown at 350 K. Moreover, the spontaneouspolarization reached the maximum value at 390 K. Since the molecularmetal oxide cluster 13 showed the P-E hysteresis, it was found that thesodium ion (Na⁺) can move between two stable ionic sites 102 a, 102 b byapplication of an electric field and the molecular polarization isreversed by movement of the sodium ion between two stable ionic sites102 a, 102 b.

FIG. 11 is a graph showing the temperature dependency of the molecularmetal oxide cluster 13 according to Example 2 in association with thespontaneous polarization thereof. In a temperature range of 350 K to 420K, a spontaneous polarization of equal to or greater than 2.0 μC/cm² wasshown, and the maximum spontaneous polarization value was shown at 390K. The molecular metal oxide cluster 13 according to Example 2 showedimprovement in a recordkeeping temperature by about 100 K as compared tothe molecular metal oxide cluster 12 including the terbium ion accordingto Example 1.

Molecular Metal Oxide Clusters Including Other Lanthanoid Ions

The molecular metal oxide cluster 13 including the sodium ion wassuccessfully synthesized, and such a molecular metal oxide cluster 13shows not only the spontaneous polarization but also the maximumspontaneous polarization value at a higher temperature than that in thetypical case. Thus, it is assumed that the level of the activationenergy when the metal ion M moves between the stable ionic sites 102 a,102 b relates to the size of the ion radius.

For this reason, molecular metal oxide clusters 10 including, as metalions M, lanthanoid ions (Gd³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺) other thana terbium ion in cluster skeletons 100 represented by a chemical formulaof P₅W₃₀O₁₁₀ was synthesized, and the electric field dependency and thetemperature dependency in association with the spontaneous polarizationwere evaluated for these molecular metal oxide clusters 10. Eachmolecular metal oxide cluster 10 including the metal ion M wassynthesized by steps similar to those of Example 1. Table 1 showssynthesis conditions for each molecular metal oxide cluster 10 includingthe lanthanoid ion. Conditions other than those shown in Table 1 aresimilar to those of synthesis of the molecular metal oxide cluster 11described in Example 1. Note that the number of water molecules cannotbe identified in the case of using Yb(NO₃)₃ as a raw material, and forthis reason, the number of mols is not described.

TABLE 1 Synthesis Compound Temperature Synthesis Name mg (mmol) (° C.)Time (h) Dy³⁺ Dy(NO₃)₃•5H₂O 84.5 (0.2) 160 24 Er³⁺ Er(NO₃)₃•5H₂O 88.4(0.2) 160 24 Gd³⁺ Gd(NO₃)₃•6H₂O 90.8 (0.2) 145 24 Ho³⁺ Ho(NO₃)₃•5H₂O132.6 (0.3) 190 72 Tm³⁺ Tm(NO₃)₃•5H₂O 53.5 (0.13) 160 24 Yb³⁺Yb(NO₃)₃•nH₂O 89.9 (—) 180 24

For the synthesized molecular metal oxide clusters, heating treatmentwas performed at the optimal heating temperature at the end. As a resultof review, it was found that the range of the optimal heatingtemperature in the heating treatment for the molecular metal oxidecluster including the lanthanoid ion is 200° C. to 250° C. The heatingtreatment is performed at this optimal heating temperature so that awater molecule present in a stable ionic site can fully disappear as inExample 1.

Relationship between Ion Radius and Activation Energy

Table 2 is a table showing the radius of the metal ion M of eachmolecular metal oxide cluster 10 and the activation energy when themetal ion M moves between the stable ionic sites 102 a, 102 b.

TABLE 2 E_(a) Ion Radius (Å) Na⁺ 1.25 1.02 Gd³⁺ 0.99 0.938 Tb³⁺ 0.960.923 Dy³⁺ 0.95 0.912 Ho³⁺ 0.92 0.901 Er³⁺ 0.87 0.890 Tm³⁺ 0.78 0.880Yb³⁺ 0.75 0.868

First, the permittivity was measured for each molecular metal oxidecluster. For each of molecular metal oxide clusters including agadolinium ion (Gd³⁺), a terbium ion (Tb³⁺), a dysprosium ion (Dy³⁺), aholmium ion (Ho³⁺), an erbium ion (Er³⁺), a thulium ion (Tm³⁺), and anytterbium ion (Yb³⁺), the heating treatment was performed at the optimalheating temperature as in Example 1, and in this manner, permittivitymeasurement values were obtained with a favorable accuracy. A peak toptemperature was read for each frequency from permittivity measurementresults obtained as in FIG. 6, and an Arrhenius plot was produced fromthe inverse of such a temperature and Ln(ω)=2πf. Note that co indicatesan angular frequency and f indicates a frequency. Note that an Arrheniusequation is the following mathematical expression and the activationenergy was calculated according to this equation.

$\begin{matrix}{{{Ln}(\omega)} = {{{- \frac{E_{a}}{k_{B}}} \cdot \frac{1}{T}} + {{Ln}\left( \omega_{0} \right)}}} & \left\lbrack {{Expression}1} \right\rbrack\end{matrix}$

Note that in the above-described mathematical expression, E_(a)indicates an activation energy, ω indicates an angular frequency, Woindicates an angular frequency at an infinite temperature, k_(B)indicates a Boltzmann constant, and T indicates a temperature. FIG. 12is a graph showing a correlation between the ion radius of the metal ionand the activation energy obtained by the above-described Arrheniusequation. With this graph, it was found that the activation energy whenthe molecular polarization of the single-molecule electret changesincreases as the ion radius of the metal ion included in the molecularmetal oxide cluster increases, and it was first found that there is thecorrelation between the ion radius and the activation energy.

The ion radius of the metal ion M included in the molecular metal oxidecluster 10 is differentiated so that the maximum temperature value thatthe spontaneous polarization is shown can be changed. In other words,for the molecular metal oxide cluster 10, the level of the activationenergy when the metal ion M moves between the stable ionic sites 102 a,102 b can be changed based on the ion radius of the metal ion M.

Note that molecular metal oxide clusters well-known so far include amolecular metal oxide cluster including a metal ion in one stable ionicsite and including a water molecule in the other stable ionic site and amolecular metal oxide cluster including a metal ion in one stable ionicsite and including no water molecule in the other stable ionic site. Formany of these typical molecular metal oxide clusters, permittivitymeasurement values cannot be obtained with a favorable accuracy, andtendency due to a difference in the metal ion cannot be analyzed. It isassumed that a reason why the permittivity measurement values cannot beobtained with a favorable accuracy is that measurement samples includethe molecular metal oxide clusters including the water molecules. Forexample, it is assumed that even when the heating treatment is performedfor the molecular metal oxide cluster described in Patent Document 1under the conditions described in Patent Document 1, the measurementvalue cannot be obtained with a favorable accuracy because the watermolecule does not fully disappear from the other stable ionic site orreturns into the other stable ionic site.

In the present embodiment, in the molecular metal oxide clusterincluding the lanthanoid ion in one stable ionic site, the watermolecule present in the other stable ionic site fully disappears.Moreover, for the molecular metal oxide cluster including the sodium ionin one stable ionic site, the optimal heating temperature at which thewater molecule present in the other stable ionic site fully disappearscan be found. Moreover, the molecular metal oxide cluster subjected tothe heating treatment at the optimal heating temperature can show thespontaneous polarization at a maximum, and the permittivity measurementvalue can be obtained with a favorable accuracy. This first leads tosuch a finding that there is the correlation between the ion radius andthe activation energy in the present embodiment.

Relationship between Ion Radius and Recordkeeping Time

The molecular metal oxide cluster 10 according to the present embodimentshows the spontaneous polarization. Thus, a single-molecule electrethaving, as a molecular structure, such a molecular metal oxide cluster10 records a change in the molecular polarization in association with asignal of 0 or 1 so that such a single-molecule electret can be appliedas a molecular memory.

In the case of including a single-molecule electret layer having, as asingle-molecule electret, each of molecular metal oxide clusters 10including a sodium ion (Na⁺), a dysprosium ion (Dy³⁺), a thulium ion(Tm³⁺), and an ytterbium ion (Yb³⁺), the recordkeeping time of such asingle-molecule electret layer as a molecular memory was calculatedusing the above-described Arrhenius equation. A temperature was −100°C., −50° C., 0° C., 27° C., 50° C., or 100° C., and the recordkeepingtime under each type of temperature environment was calculated. Resultsare shown in Table 3 and FIG. 13.

TABLE 3 Ion Temperature Recordkeeping Time (s) Radius (° C.) −100 −50 027 50 100 (Å) Na⁺ 9.1 × 10²²  8.8 × 10¹³ 1.7 × 10⁸ 8.7 × 10⁵ 1.9 × 10⁴2.5 × 10¹  1.02 Dy³⁺ 2.4 × 10¹³ 1.5 × 10⁷ 1.7 × 10³ 4.5 × 10¹ 3.3 × 10⁰3.4 × 10⁻² 0.912 Tm³⁺ 9.6 × 10¹⁰ 7.3 × 10⁵ 4.2 × 10² 2.1 × 10¹ 2.4 × 10⁰5.5 × 10⁻² 0.880 Yb³⁺ 1.5 × 10¹⁰ 2.0 × 10⁵ 1.5 × 10² 8.8 × 10⁰ 1.1 × 10⁰3.0 × 10⁻² 0.868

As shown in Table 3, the molecular memories including, as thesingle-molecule electrets, the molecular metal oxide clusters 10including the sodium ion (Na⁺), the dysprosium ion (Dy³⁺), the thuliumion (Tm³⁺), and the ytterbium ion (Yb³⁺) showed greatly-differentrecordkeeping temperatures according to the included metal ion. Fromthese results, it was found that in the molecular memory including, asthe single-molecule electret, the molecular metal oxide clusteraccording to the present embodiment, the recordkeeping time in atemperature range of −100° C. to 100 ° C. based on the ion radius of themetal ion is in an extremely-wide range of 3.0×10⁻² seconds to 9.1×10²²seconds.

Moreover, as shown in FIG. 13, it was found that the recordkeeping timeincreases as the ion radius of the metal ion increases and there is acorrelation between the ion radius of the metal ion and therecordkeeping time of the molecular memory.

Specifically, the recordkeeping time of the molecular memory using thesingle-molecule electret of the present embodiment at −100° C. was in arange of 1.5×10¹⁰ seconds to 9.1×10²² seconds. As described above, sincethe molecular memory of the present embodiment has an extremely-longrecordkeeping time under an environment of −100° C., such a molecularmemory is suitable for a non-volatile memory.

The recordkeeping time of the molecular memory using the single-moleculeelectret of the present embodiment at −50° C. was in a range of 2.0×10⁵seconds to 8.8×10¹³ seconds. Among these molecular memories, thesingle-molecule electret including the sodium ion (Na⁺) has anextremely-long recordkeeping time (8.8×10¹³ seconds), and therefore, issuitable for a non-volatile memory. The single-molecule electretsincluding the dysprosium ion (Dy³⁺), the thulium ion (Tm³⁺), and theytterbium ion (Yb³⁺) have a relatively-long recordkeeping time of abouttwo days to about six months, and therefore, are suitable fornon-volatile memories or storage class memories.

The recordkeeping time of the molecular memory using the single-moleculeelectret of the present embodiment at 0° C. was in a range of 1.5×10²seconds to 1.7×10⁸ seconds. Among these molecular memories, thesingle-molecule electret including the sodium ion (Na⁺) has a longrecordkeeping time (about five years), and therefore, is suitable for anon-volatile memory or a storage class memory. The single-moleculeelectrets including the dysprosium ion (Dy³⁺), the thulium ion (Tm³⁺),and the ytterbium ion (Yb³⁺) have a slightly-short recordkeeping time ofabout three minutes to about 30 minutes, and therefore, are suitable forvolatile memories or storage class memories.

The recordkeeping time of the molecular memory using the single-moleculeelectret of the present embodiment at 27° C. was in a range of 8.8×10°seconds to 8.7×10⁵ seconds. Among these molecular memories, thesingle-molecule electret including the sodium ion (Na⁺) has aslightly-long recordkeeping time (about 10 days), and therefore, isassumed to be suitable for a non-volatile memory or a storage classmemory. The single-molecule electrets including the dysprosium ion(Dy³⁺), the thulium ion (Tm³⁺), and the ytterbium ion (Yb³⁺) have ashort recordkeeping time of about nine seconds to about eight minutes,and therefore, are suitable for volatile memories or storage classmemories.

The recordkeeping time of the molecular memory using the single-moleculeelectret of the present embodiment at 50° C. was in a range of 1.1×10°seconds to 1.9×10⁴ seconds. Among these molecular memories, thesingle-molecule electret including the sodium ion (Na⁺) has aslightly-short recordkeeping time (about five hours), and therefore, issuitable for a non-volatile memory or a storage class memory. Thesingle-molecule electrets including the dysprosium ion (Dy³⁺), thethulium ion (Tm³⁺), and the ytterbium ion (Yb³⁺) have a shortrecordkeeping time of about one second to about three seconds, andtherefore, are suitable for volatile memories or storage class memories.

The recordkeeping time of the molecular memory using the single-moleculeelectret of the present embodiment at 100° C. was in a range of 3.0×10⁻²seconds to 2.5×10¹ seconds. Among these molecular memories, thesingle-molecule electret including the sodium ion (Na⁺) has aslightly-short recordkeeping time (about four minutes), and therefore,is assumed to be suitable for a non-volatile memory or a storage classmemory. The single-molecule electrets including the dysprosium ion(Dy³⁺), the thulium ion (Tm³⁺), and the ytterbium ion (Yb³⁺) have anextremely-short recordkeeping time of equal to or shorter than onesecond, and therefore, are suitable for volatile memories.

Note that the above-described uses of the molecular memory are merelypreferred examples and are not limited to above. The molecular memoriesincluding the single-molecule electret layers having thesesingle-molecule electrets can be used for any of a volatile memory, anon-volatile memory, and a storage class memory based on therecordkeeping time.

Control of Recordkeeping Time by Oxidation-Reduction Reaction

Next, for checking a change in the properties of the molecular memory inthe case of changing an electronic state of the single-molecule electretaccording to the present embodiment, oxidation reaction and reductionreaction were made for the molecular metal oxide cluster 12 includingterbium.

The cluster skeleton 100 of the molecular metal oxide cluster 12obtained by the above-described method includes 30 atoms of hexavalenttungsten (W). Of these tungsten atoms, eight atoms were reduced, and inthis manner, a reduced molecular metal oxide cluster 12 b havingpentavalent tungsten was obtained. An original molecular metal oxidecluster of the reduced molecular metal oxide cluster 12 b will bereferred to as an oxidized molecular metal oxide cluster 12 a. FIG. 14shows a schematic diagram of such reaction and photographs of themolecular metal oxide clusters 12 a, 12 b before and after the reaction.The oxidation-reduction reaction of the molecular metal oxide cluster 12is reversible, and the reduced molecular metal oxide cluster 12 b can bechanged back to the oxidized molecular metal oxide cluster 12 a by theoxidation reaction. Note that the reduction reaction of the molecularmetal oxide cluster 12 was made by the following steps.

Reduction Reaction of Molecular Metal Oxide Cluster 12

First, 4 ml of H₂O was added to 0.1 g (1.2×10⁻⁵ mol) of the molecularmetal oxide cluster 12, and the solution was heated to 75° C. Next, 1.38ml (2.8×10⁻³ mol) of hydrazine monohydrate was added to the solution.After bubbling with nitrogen gas for five minutes, the solution washeated at 100° C. for 15 minutes. After heating, the solution was cooledto a room temperature. After half a day, a perse single crystal wasprecipitated. In the above-described manner, the reduced molecular metaloxide cluster 12 b was obtained.

A single crystal of the oxidized molecular metal oxide cluster 12 a wasobtained in such a manner that the reduced molecular metal oxide cluster12 b stands still at a room temperature for one week or is heated at120° C. for half a day.

For the resultant reduced molecular metal oxide cluster 12 b, thepermittivity was measured by the above-described method, and thefrequency dependency and the temperature dependency in association withthe permittivity were checked. Results are shown in FIG. 15. Themeasurement results for the reduced molecular metal oxide cluster 12 bshown in FIG. 15 were greatly different from the measurement results forthe oxidized molecular metal oxide cluster 12 a shown in FIG. 6. Atemperature (the blocking temperature) when movement of the metal ion isassumed to be stopped (a movement speed of 0.1 Hz) was 286 K in the caseof the oxidized molecular metal oxide cluster 12 a and 254 K in the caseof the reduced molecular metal oxide cluster 12 b.

Further, for the molecular metal oxide cluster 13 including the sodiumion according to Example 2, the reduction reaction was made by a methodsimilar to the above-described method. Eight atoms of hexavalenttungsten in the cluster skeleton 100 were reduced to pentavalenttungsten, and in this manner, a reduced molecular metal oxide cluster 13b was obtained. An original molecular metal oxide cluster of the reducedmolecular metal oxide cluster 13 b will be referred to as an oxidizedmolecular metal oxide cluster 13 a. For checking theoxidization-reduction reaction properties of the molecular metal oxidecluster 13, measurement by cyclic voltammetry was performed.

The oxidized molecular metal oxide cluster 13 a and the reducedmolecular metal oxide cluster 13 b were adjusted such that a sampleconcentration in a solution is adjusted to 1 mmol/L, and were dissolvedin 1 mol/L of an HCl water solution. A glass-like carbon electrode wasused as a working electrode in the cyclic voltammetry, and a platinumelectrode and a silver/silver chloride electrode were used as a counterelectrode and a reference electrode. Results of measurement in a rangeof 0.6 V to +0.4 V are shown in FIG. 16. A current-potential curve ofFIG. 16 showed that the oxidation-reduction reaction of the molecularmetal oxide cluster 13 is reversible.

From permittivity measurement results obtained for the oxidizedmolecular metal oxide clusters 12 a, 13 a and the reduced molecularmetal oxide clusters 12 b, 13 b, the recordkeeping time of a molecularmemory including a single-molecule electret layer having each of theseclusters as a single-molecule electret was calculated using theabove-described Arrhenius equation. Results are shown in Table 4.

TABLE 4 13a 13b 12a 12b E_(a) 1.380 0.62 0.960 0.55 E_(a)/R 16014 719211136 6380 Ln(ω₀) 41.543 24.950 38.474 24.654 Ion Radius (Å) 1.02 1.020.923 0.923 Recordkeeping −100 (° C.)   9.1 × 10²² 1.0 × 10⁸ 1.1 × 10¹²1.3 × 10⁶  Time (second) −50 (° C.)  8.8 × 10¹³ 9.3 × 10³ 6.0 × 10⁵  3.3× 10²   0 (° C.) 1.7 × 10⁸ 2.5 × 10¹ 6.4 × 10   1.7 × 10⁰   27 (° C.)8.7 × 10⁵ 2.4 × 10⁰ 1.6 × 10⁰  2.1 × 10⁻¹  50 (° C.) 1.9 × 10⁴  4.3 ×10⁻¹ 1.2 × 10⁻¹ 4.7 × 10⁻² 100 (° C.) 2.5 × 10¹  2.2 × 10⁻² 1.1 × 10⁻³3.3 × 10⁻³

The reduced molecular metal oxide cluster 13 b obtained in such a mannerthat the electronic state is changed by the reduction reaction of themolecular metal oxide cluster 13 including the sodium ion had arecordkeeping time of 2.4×10° seconds at 27° C. This recordkeeping timeis extremely shorter than a recordkeeping time of 8.7×10⁵ seconds (about10 days) at 27° C. in the oxidized molecular metal oxide cluster 13 abefore the reduction reaction. Under other temperature conditions, therecordkeeping time greatly varied before and after the reductionreaction. For the oxidized molecular metal oxide clusters 12 a, 12 bincluding the terbium ions, results showing a great difference in therecordkeeping time were similarly obtained. From these results, it wasfound that by a change in the electronic state of the single-moleculeelectret of the present embodiment by the oxidization reaction or thereduction reaction, the molecular memory using the single-moleculeelectret after the reaction has a recordkeeping time different from thatin the case of using the single-molecule electret before the reaction.Note that the reduction reaction of the present embodiment is made dueto reduction in some atoms of the cluster skeleton and similar resultsare assumed to be obtained from single-molecule electrets having thesame cluster skeleton or similar cluster skeletons.

Method for Manufacturing Molecular Memory with ControllableRecordkeeping Time

As described above, for the single-molecule electret of the presentembodiment, the recordkeeping time is controllable according to themetal ion to be included and the temperature range. As further describedabove, the recordkeeping time can be significantly changed by a changein the electronic state of the single-molecule electret by theoxidization reaction or the reduction reaction. Thus, thesingle-molecule electret according to the embodiment of the presentinvention is, as a molecular memory including a single-molecule electretlayer having such a single-molecule electret and recording the molecularpolarization of the single-molecule electret in association with asignal of 0 or 1, applicable to various electronic devices, and amolecular memory having a recordkeeping time suitable for an intendeduse can be manufactured.

The single-molecule electret used for manufacturing the molecular memoryaccording to the embodiment of the present invention includes, asdescribed above, the cluster skeleton having the continuous hole and theplurality of stable ionic sites provided apart from each other in thecontinuous hole and the metal ion included in any one of the stableionic sites and formed movable to the other hollow stable ionic site.Moreover, the single-molecule electret shows the molecular polarizationin a state in which the metal ion is included in any one of the stableionic sites, and when an electric field is applied, the metal ion movesto the other hollow stable ionic site such that the molecularpolarization is changed.

The molecular memory according to the embodiment of the presentinvention has the controllable recordkeeping time. The method formanufacturing such a molecular memory will be described based on aflowchart shown in FIG. 17.

In Step 1 after the start, the use of the molecular memory is specifiedas any of a volatile memory, a storage class memory, and a non-volatilememory.

In Step 2, the approximate recordkeeping time of the molecular memory isdetermined based on the specified use.

In Step 3, the metal ion is selected based on the determinedrecordkeeping time. Note that such a recordkeeping time is a guide forselecting the metal ion and may have, e.g., a range of about one secondto about 10 seconds or a range of about one month to about three months.

In Step 4, the selected metal ion is included in the cluster skeleton,and in this manner, the single-molecule electret whose molecularpolarization is changeable is obtained.

Step 5 is the oxidization-reduction step of changing the recordkeepingtime by changing the electronic state of the resultant single-moleculeelectret by the oxidization reaction or the reduction reaction.

In Step 6, the single-molecule electret layer is formed using thesingle-molecule electret, and is mounted on the molecular memory.

Note that Step 5 may be omitted and Step 6 may be performed next afterStep 4.

For example, the single-molecule electret layer including, as themolecular structure of the single-molecule electret of the presentembodiment, the molecular metal oxide cluster 10 may be mounted on afield-effect transistor. When voltage is applied to a gate electrode ofthe field-effect transistor, the single-molecule electret layer ispolarized, and a channel region is formed. The channel region ismaintained even after the power has been turned off, and therefore, sucha single-molecule electret layer can be used for a non-volatile memory.Since the molecular metal oxide cluster 13 including sodium has a highactivation energy when the sodium ion moves between the stable ionicsites 102 a, 102 b, the recordkeeping temperature can be drasticallyimproved as compared to the typical case if the molecular metal oxidecluster 13 is, as the single-molecule electret layer, mounted on thefield-effect transistor.

A volatile memory including the single-molecule electret layer havingthe molecular metal oxide cluster 10 as the molecular structure of thesingle-molecule electret of the present embodiment is also useful. Inthe case of mounting the single-molecule electret layer including themolecular metal oxide cluster 10 on the volatile memory, the electricfield response speed of the volatile memory varies according to the ionradius of the metal ion included in the cluster skeleton. The molecularmetal oxide cluster 10 including the metal ion with a relatively-smallion radius is mounted so that the electric field response speednecessary for the volatile memory can be improved.

EXAMPLE 3

Next, an example where a molecular metal oxide cluster 12 including aterbium ion is applied to a volatile memory and a non-volatile memorywill be described below as Example 3.

A single-molecule electret layer including the molecular metal oxidecluster 12 according to the present embodiment can record the molecularpolarization of the molecular metal oxide cluster 12 in association witha signal of 0 or 1, and can be mounted on a capacitor of the volatilememory or the non-volatile memory. This single-molecule electret layeris formed between, e.g., capacitor electrodes by application of themolecular metal oxide cluster 12. The single-molecule electret layer ispolarized between the electrodes, and such polarization is associatedwith a signal of 0 or 1. In this manner, data is recorded. The molecularmetal oxide cluster 12 according to the present embodiment shows, onlywith one molecule, one bit as a basic information amount unit, andtherefore, the capacity of a storage device can be increased.

FIG. 18 is a block diagram for describing the outline of the storagedevice using the volatile memory and the non-volatile memory accordingto the present embodiment. For example, as shown in FIG. 18, the storagedevice 200 has a power control unit 201, a storage unit 202, a CPU 203,an input unit 204, and an output unit 205. The storage unit 202 has thenon-volatile memory 207 and the volatile memory 206, and exchanges datawith the outside via the input unit 204 and the output unit 205. The CPU203 has a control unit 208 and an arithmetic processing unit 209.

The volatile memory 206 is, for example, a dynamic random access memory(DRAM). The volatile memory 206 loses the stored data over time as longas the volatile memory 206 is not refreshed by an external electricfield, but has an advantage that a wiring speed and a reading speed arefast. The single-molecule electret layer including the molecular metaloxide cluster 12 according to the present embodiment is mounted on thecapacitor of the volatile memory 206 so that the writing speed and thereading speed can be controlled according to the ion radius of theincluded metal ion.

The non-volatile memory 207 is, for example, a flash memory. Thesingle-molecule electret layer including the molecular metal oxidecluster 12 according to the present embodiment is mounted on thecapacitor of the non-volatile memory 207 so that a recordkeepingtemperature can be controlled according to the ion radius of theincluded metal ion.

In the present embodiment, the single-molecule electret layer includingthe molecular metal oxide cluster is mounted on each of the volatilememory 206 and the non-volatile memory 207 to form the storage device,but is not necessarily mounted on each of the volatile memory and thenon-volatile memory. Even if the single-molecule electret layerincluding the molecular metal oxide cluster of the present embodiment ismounted on any of the volatile memory and the non-volatile memory, theabove-described advantageous effects can be obtained.

FIG. 19 is a circuit diagram showing one example of a memory cell of thevolatile memory according to Example 3. Writing and Reading operation isperformed in such a manner that current flows in a word line 410 and abit line 411 arranged in a grid pattern. The memory cell has atransistor 412 and a capacitor 413 connected to the transistor 412 inseries. The gate and drain of the transistor 412 are each connected tothe word line 410 and the bit line 411. The capacitor 413 includes,between electrodes, the single-molecule electret layer formed of themolecular metal oxide cluster 12, and the data is written and read suchthat the polarization of the single-molecule electret layer isassociated with a signal of 0 or 1.

Features and Effects of Examples

As described above, according to the molecular metal oxide cluster 10 ofthe present embodiment, when an electric field is applied, the metal ionM moves between two stable ionic sites 102 a, 102 b, and accordingly,the molecular polarization is reversed. The relaxation time at thispoint is controllable by the temperature, and therefore, the temperaturecontrol allows application to various electronic devices with differentresponse speeds. Further, for the molecular metal oxide cluster of thepresent embodiment, the maximum temperature value that the spontaneouspolarization is shown varies according to the ion radius of the includedmetal ion M. That is, the level of the activation energy upon reversalof the polarization varies according to the ion radius of the includedmetal ion M. Thus, by the temperature control and selection of the metalion M to be included, the molecular metal oxide cluster of the presentembodiment can exhibit excellent properties suitable for each use uponapplication to various electronic devices.

In the molecular memory of the present embodiment, the recordkeepingtime in a temperature range of −100° C. to 100° C. based on the ionradius of the metal ion included in the stable ionic site of the clusterskeleton is an extremely-wide range of 3.0×10⁻² seconds to 9.1×10²²seconds. Thus, based on the metal ion and the temperature environment,the recordkeeping time of the molecular memory can be controlled withinan extremely-wide range. The volatile memory, the non-volatile memory,and the storage class memory each have recordkeeping times suitabletherefor. Thus, the metal ion and the temperature range can be, forproviding the molecular memory having the recordkeeping time suitablefor the memory use, selected targeted for any of the memory usesincluding the volatile memory, the non-volatile memory, and the storageclass memory.

In a case where the metal ion with a relatively-large ion radius isincluded, the recording stability and recordkeeping temperature of thenon-volatile memory can be improved and the reliability of the capacitoretc. can be ensured by means of the properties that the activationenergy is high upon reversal of the polarization, for example.Particularly, in a case where the included metal ion M is the sodiumion, a spontaneous polarization of equal to or greater than 2.0 μC/cm²is shown in a temperature range of 350 K to 420 K, and the maximumspontaneous polarization value is shown at a higher temperature thanthat in the typical case. Thus, the molecular metal oxide cluster 13including sodium can drastically improve the reliability of theelectronic device.

In a case where the metal ion with a relatively-small ion radius isincluded, the response speed of an actuator can be improved and thesensitivities of various sensors can be improved by means of theproperties that the activation energy is low upon reversal of thepolarization, for example.

Other Molecular Structures of Single-Molecule El ectret

In the above-described embodiment, the Preyssler-type polyoxometalateskeleton as shown in FIGS. 1 and 2 is used as the cluster skeleton 100,but the present invention is not limited to the cluster skeleton 100. Itis suggested that in a case where the metal ion is delocalized(disorder) in the cluster skeleton, the molecular metal oxide clusterhaving the continuous hole and the stable ionic sites provided apartfrom each other in the continuous hole shows the molecular polarizationas in Preyssler-type POM. It is assumed that such a molecular metaloxide cluster is, as the single-molecule electret, applicable to themolecular memory as in the above-described embodiment and shows similarproperties. Note that in the present specification, del ocalizationindicates dynamic disorder.

FIGS. 20, 21, and 24 to 37 are schematic diagrams of a molecular metaloxide cluster having a polyoxometalate skeleton as a cluster skeleton asin Preyssler-type POM, delocalization (disorder) of a metal ion M in thecluster skeleton being confirmed. Note that for the sake of simplicityin description, FIGS. 20, 21, and 24 to 37 show a state in which themetal ion M is in any of stable ionic sites, but the metal ion M isactually delocalized (disorder) between the stable ionic sites.

The molecular metal oxide cluster 14 shown in FIGS. 20 and 21 has, as acluster skeleton 140, a heart-like polyoxometalate skeleton deformedsuch that part of the cluster skeleton 100 is recessed. The clusterskeleton 140 is in the shape of a substantially flat spheroid formedshort in an axial direction and long in a radial direction as in theabove-described cluster skeleton 100, and has one continuous hole 141extending along a rotation axis. The continuous hole 141 has stableionic sites 142 a, 142 b on one and other opening end sides, and onepotassium ion (K⁺) is, as the metal ion M, included in the continuoushole 141. The chemical formula of the molecular metal oxide cluster 14is represented by [K⁺:{W₂CoO₈(H₂O)₂}(P₂W₁₂O₄₆)₂]¹⁹⁻. The potassium ionis delocalized (disorder) in the axial direction.

Synthesis of Molecular Metal Oxide Cluster 14

First, K₁₂[H₂P₂W₁₂O₄₈].24H₂O was synthesized by an already-reportedmethod. Next, 2.0 g (0.51 mmol) of synthesized K₁₂[H₂P₂W₁₂O₄₈].24H₂O wasdissolved in a water-glacial acetic acid mixture water solution (avolume ratio of 2:1), and in this manner, a first solution was prepared.Next, 6 ml of a cobalt chloride water solution (1 mol/L) and 4 ml of asodium chloride water solution (1 mol/L) were mixed to prepare a secondsolution. The second solution was dropped into the first solution duringstirring, and the resultant was refluxed for 10 hours. After refluxing,the first solution was cooled to a room temperature, and the crystalpowder of the molecular metal oxide cluster 14 was obtained (see ZhimingZhang, Shuang Yao, Yanfei Qi, Yangguang Li, Yonghui Wang, Enbo Wang,Dalton Transactions, 2008, 3051 - 3053).

After the frequency dependency and the temperature dependency inassociation with the permittivity had been measured for the molecularmetal oxide cluster 14, results shown in FIG. 22 were obtained. As inthe molecular metal oxide cluster 12 including the terbium ion,frequency dispersion was confirmed in a low-temperature range of equalto or lower than 400 K. In a high-temperature range, a molecularpolarization relaxation phenomenon was confirmed in the case of afrequency of 100 Hz. Since such a range is a higher temperature rangethan a measurement limit temperature, no relaxation phenomenon wasconfirmed in the case of other frequencies, but it is assumed thattendency similar to that of the molecular metal oxide cluster 10 isshown.

For the molecular metal oxide cluster 14, a P-E hysteresis was measured.FIG. 23 is a P-E hysteresis curve showing the electric field dependencyof the molecular metal oxide cluster 14 in association with thepolarization thereof. After the temperature had increased from 294 K to371 K, the P-E hysteresis was shown, and an increase in the spontaneouspolarization was confirmed. Note that 365 K is the highest temperaturefor the spontaneous polarization. From these results, it is also assumedthat the molecular metal oxide cluster 14 exhibits physical propertiessimilar to those of the molecular metal oxide cluster 10.

FIGS. 24 and 25 show a molecular metal oxide cluster 15 having, as acluster skeleton 150, a polyoxometalate skeleton in such a shape thatthe above-described cluster skeleton 100 is extended in acircumferential direction. The cluster skeleton 150 includes, in thecircumferential direction, four continuous holes 151 extending in adirection substantially parallel with the rotation axis of the clusterskeleton 150. Each continuous hole 151 has stable ionic sites 152 a, 152b on one and the other opening end sides, and one metal ion M isincluded in each continuous hole 151. The chemical formula of themolecular metal oxide cluster 15 is represented by [M⁺4:H₇P₈W₄₈O₁₈₄]²⁹⁻.Each metal ion M is delocalized (disorder) in an axial direction, andtherefore, it is assumed that physical properties similar to those ofthe molecular metal oxide cluster 10 are exhibited.

A molecular metal oxide cluster 16 shown in FIG. 26 has, as a clusterskeleton 160, a polyoxometalate skeleton in such a shape that twocluster skeletons 100 are coupled substantially perpendicularly to eachother. The cluster skeleton 160 includes two continuous holes 161extending in directions substantially perpendicular to each other. Eachcontinuous hole 161 has stable ionic sites 162 a, 162 b on one and theother opening end sides, and one metal ion M is included in eachcontinuous hole 161. The chemical formula of the molecular metal oxidecluster 16 is represented by [{Sn(CH₃)₂}4(M⁺:H₂P₄W₂₄O₉₂)₂]²⁶⁻. The metalion M is delocalized (disorder) in two directions substantiallyperpendicular to each other, and therefore, it is assumed that physicalproperties similar to those of the molecular metal oxide cluster 10 areexhibited.

A molecular metal oxide cluster 17 shown in FIGS. 27 and 28 has, as acluster skeleton 170, a polyoxometalate skeleton in such a shape thatpart of the cluster skeleton 100 is cleaved and a phenyl group ismodified at the terminal of the cleaved cluster skeleton 100. Thecluster skeleton 170 has one continuous hole 171 as in the clusterskeleton 100. The continuous hole 171 has stable ionic sites 172 a, 172b on one and the other opening end sides, and one metal ion M isincluded in the continuous hole 171. The chemical formula of themolecular metal oxide cluster 17 is represented by[M⁺:(PhPO)₂P₄W₂₄O₉₂]¹⁵⁻ or [M⁺:(PhAsO)₂P₄W₂₄O₉₂]¹⁵⁻. The metal ion M isdelocalized (disorder) in an axial direction, and therefore, it isassumed that physical properties similar to those of the molecular metaloxide cluster 10 are exhibited.

A molecular metal oxide cluster 18 shown in FIGS. 29 and 30 has, as acluster skeleton 180, a polyoxometalate skeleton in such a shape thatpart of the cluster skeleton 100 is cleaved and a phenyl group ismodified at the terminal of the cleaved cluster skeleton 100. Thecluster skeleton 180 has one continuous hole 181 as in the clusterskeleton 100. The continuous hole 181 has stable ionic sites 182 a, 182b on one and the other opening end sides, and one metal ion M isincluded in the continuous hole 181. The chemical formula of themolecular metal oxide cluster 18 is represented by[M⁺:{Co(H₂O)₄}₂(PhPO)₂P₄W₂₄O₉₂]⁹⁻. The metal ion M is delocalized(disorder) in an axial direction, and therefore, it is assumed thatphysical properties similar to those of the molecular metal oxidecluster 10 are exhibited.

A molecular metal oxide cluster 19 shown in FIGS. 31 and 32 has, as acluster skeleton 190, a polyoxometalate skeleton in the shape of such asubstantially flat spheroid that the cluster skeleton 100 is extended ina circumferential direction. The cluster skeleton 190 has threecontinuous holes 191 in a direction substantially perpendicular to therotation axis of the cluster skeleton 190. Each continuous hole 191 hasstable ionic sites 192 a, 192 b on one and the other opening end sides,and one metal ion M is included in each continuous hole 191. Thechemical formula of the molecular metal oxide cluster 19 is representedby [K⁺:P₈W₄₈O₁₈(H₄W₄O₁₂)₂}Ln₂(H₂O)₁₀]²⁵⁻ (Ln=La, Ce, Pr, Nd). The metalions M included in the molecular metal oxide cluster 19 are specificallyone potassium ion and two lanthanoid ions. The metal ion M isdelocalized (disorder) in the direction substantially perpendicular tothe rotation axis, and therefore, it is assumed that physical propertiessimilar to those of the molecular metal oxide cluster 10 are exhibited.

A molecular metal oxide cluster 20 shown in FIGS. 33 and 34 has, as acluster skeleton 120, a polyoxometalate skeleton in the shape of such asubstantially flat spheroid that the cluster skeleton 100 is extended ina circumferential direction. The cluster skeleton 120 has fivecontinuous holes 121 in a direction substantially parallel with therotation axis of the cluster skeleton 120 and a direction substantiallyperpendicular to the rotation axis. Each continuous hole 121 has stableionic sites 122 a, 122 b on one and the other opening end sides, and onemetal ion M is included in each continuous hole 121. The chemicalformula of the molecular metal oxide cluster 20 is represented by [K⁺ ₃:P₈W₄₈O₁₈(H₄W₄O₁₂)₂}Ce₂(H₂O)₁₀]²⁵⁻. The metal ions M included in themolecular metal oxide cluster 20 are specifically three potassium ionsand two cerium ions. Of three potassium ions, two potassium ions aredelocalized (disorder) in the direction substantially parallel with therotation axis, and the remaining one potassium ion and two cerium ionsare delocalized (disorder) in the direction substantially perpendicularto the rotation axis. Thus, it is assumed that physical propertiessimilar to those of the molecular metal oxide cluster 10 are exhibited.

A molecular metal oxide cluster 21 shown in FIGS. 35 and 36 has, as acluster skeleton 210, a polyoxometalate skeleton in the shape of such asubstantially flat spheroid that the cluster skeleton 100 is extended ina circumferential direction. The cluster skeleton 210 has four metalions M (lanthanoid ions), and two stable ionic sites 212 a, 212 bprovided apart from each other for each metal ion M are present. Themolecular metal oxide cluster 21 has the total of eight stable ionicsites, and four metal ions M are randomly delocalized (disorder) amongthese eight stable ionic sites. Moreover, each metal ion M is furtherdelocalized (disorder) in the stable ionic sites 212 a, 212 b. Thus, itis assumed that the molecular metal oxide cluster 21 exhibits, byvoltage application, physical properties similar to those of themolecular metal oxide cluster 10. Note that the chemical formula of themolecular metal oxide cluster 21 is represented by {[Ln³⁺₂(μ-OH)₄(H₂O)_(x)]₂[H₂₄P₈W₄₈O₁₈₄]}¹²⁻(Ln=Nd, Sm, Tb).

In these molecular metal oxide clusters 14, 15, 16, 17, 18, 19, 20, 21having the polyoxometalate skeletons, the metal ions M are delocalized(disorder) among the stable ionic sites, and therefore, effects similarto those of the above-described molecular metal oxide cluster 10 areexpected. Note that the metal ion M is not limited to those describedabove and other metal ions M may be used as long as the metal ion M isdelocalized (disorder) in the cluster skeleton.

A molecular metal oxide cluster 22 shown in FIG. 37 has a clusterskeleton 220, and includes iodine as a metal ion. The chemical formulaof the molecular metal oxide cluster 22 is represented by[H₃W₁₈O₅₆(IO₆)]⁶⁻. When two tungsten atoms of the cluster skeleton 220are reduced from hexavalent tungsten to pentavalent tungsten by thereduction reaction, coordination environment around iodine at the centerof the cluster skeleton 220 changes, and accordingly, iodine displacesto one side of the cluster skeleton 220. The position of the metal ion Mis controlled by the oxidization-reduction reaction, and therefore,effects similar to those of the above-described molecular metal oxidecluster 10 are expected.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, the metal ion tobe included is selected so that the molecular metal oxide cluster can beapplied to various electronic devices.

DESCRIPTION OF REFERENCE CHARACTERS

-   10 Molecular Metal Oxide Cluster-   11 Molecular Metal Oxide Cluster-   12 Molecular Metal Oxide Cluster-   13 Molecular Metal Oxide Cluster-   100 Cluster Skeleton-   101 Continuous Hole-   102 a Stable Ionic Site-   102 b Stable Ionic Site-   200 Storage Device-   206 Volatile Memory-   207 Non-Volatile Memory-   M Metal Ion

1. A molecular memory including a single-molecule electret layer havinga single-molecule electret showing molecular polarization and recordingthe molecular polarization of the single-molecule electret inassociation with a signal of 0 or 1, the single-molecule electretincluding a cluster skeleton having a continuous hole and a plurality ofstable ionic sites provided apart from each other in the continuous holeand a metal ion included in any one of the stable ionic sites and formedmovable to the other hollow stable ionic site, the molecularpolarization being shown in a state in which the metal ion is includedin any one of the stable ionic sites, the molecular polarization beingchanged by movement of the metal ion to the other hollow stable ionicsite by application of an electric field, a recordkeeping time of themolecular memory in a temperature range of −100° C. to 100° C. based onan ion radius of the metal ion being 3.0×10⁻² seconds to 9.1×10²²seconds, based on the recordkeeping time, the molecular memory beingused as any of a volatile memory, a non-volatile memory, and a storageclass memory.
 2. The molecular memory of claim 1, wherein by a change inan electronic state of the single-molecule electret by oxidizationreaction or reduction reaction, the recordkeeping time in the case ofusing the single-molecule electret after the reaction is different fromthe recordkeeping time in the case of using the single-molecule electretbefore the reaction.
 3. The molecular memory of claim 1 or 2, whereinthe cluster skeleton is a polyoxometalate skeleton, and the metal ion isdelocalized in the cluster skeleton.
 4. The molecular memory of claim 3,wherein the cluster skeleton is a polyoxometalate skeleton having twostable ionic sites and represented by a chemical formula of P₅W₃₀O₁₁₀ or{W₂Co₂₀₈(H₂O)₂}P₄W₂₄O₉₂.
 5. The molecular memory of claim 4, wherein thecluster skeleton is a polyoxometalate skeleton having two stable ionicsites and represented by a chemical formula of P₅W₃₀O₁₁₀, the metal ionis one type selected from a group consisting of a sodium ion (Na⁺), agadolinium ion (Gd³⁺), a terbium ion (Tb³⁺), a dysprosium ion (Dy³⁺), aholmium ion (Ho³⁺), an erbium ion (Er³⁺), a thulium ion (Tm³⁺), and anytterbium ion (Yb³⁺), and as the ion radius of the metal ion increases,an activation energy when the molecular polarization of thesingle-molecule electret changes increases.
 6. The molecular memory ofclaim 4, wherein the cluster skeleton is a polyoxometalate skeletonhaving two stable ionic sites and represented by a chemical formula ofP₅W₃₀O₁₁₀, the metal ion is one type selected from a group consisting ofa sodium ion (Na⁺), a dysprosium ion (Dy³⁺), a thulium ion (Tm³⁺), andan ytterbium ion (Yb³⁺), and as the ion radius of the metal ionincreases, the recordkeeping time increases.
 7. The molecular memory ofclaim 6, wherein the cluster skeleton is a polyoxometalate skeletonhaving two stable ionic sites and represented by a chemical formula ofP₅W₃₀O₁₁₀, the metal ion is one type selected from a group consisting ofa sodium ion (Na⁺), a dysprosium ion (Dy³⁺), a thulium ion (Tm³⁺), andan ytterbium ion (Yb³⁺), and the recordkeeping time at −100° C. is1.5×10¹⁰ seconds to 9.1×10²² seconds.
 8. The molecular memory of claim6, wherein the cluster skeleton is a polyoxometalate skeleton having twostable ionic sites and represented by a chemical formula of P₅W₃₀O₁₁₀,the metal ion is one type selected from a group consisting of a sodiumion (Na⁺), a dysprosium ion (Dy³⁺), a thulium ion (Tm³⁺), and anytterbium ion (Yb³⁺), and the recordkeeping time at −50° C. is 2.0×10⁵seconds to 8.8×10¹³ seconds.
 9. The molecular memory of claim 6, whereinthe cluster skeleton is a polyoxometalate skeleton having two stableionic sites and represented by a chemical formula of P₅W3o0110, themetal ion is one type selected from a group consisting of a sodium ion(Na⁺), a dysprosium ion (Dy³⁺), a thulium ion (Tm³⁺), and an ytterbiumion (Yb³⁺), and the recordkeeping time at 27° C. is 8.8×10° seconds to8.7×10⁵ seconds.
 10. The molecular memory of claim 6, wherein thecluster skeleton is a polyoxometalate skeleton having two stable ionicsites and represented by a chemical formula of P₅W₃₀O₁₁₀, the metal ionis one type selected from a group consisting of a sodium ion (Na⁺), adysprosium ion (Dy³⁺), a thulium ion (Tm³⁺), and an ytterbium ion(Yb³⁺), and the recordkeeping time at 50° C. is 1.1×10° seconds to1.9×10⁴ seconds.
 11. The molecular memory of claim 6, wherein thecluster skeleton is a polyoxometalate skeleton having two stable ionicsites and represented by a chemical formula of P₅W₃₀O₁₁₀, the metal ionis one type selected from a group consisting of a sodium ion (Na⁺), adysprosium ion (Dy³⁺), a thulium ion (Tm³⁺), and an ytterbium ion(Yb³⁺), and the recordkeeping time at 100° C. is 3.0×10⁻² seconds to2.5×10′ seconds.
 12. A method for manufacturing a molecular memoryincluding a single-molecule electret layer having a single-moleculeelectret showing molecular polarization and recording the molecularpolarization of the single-molecule electret in association with asignal of 0 or 1, the single-molecule electret including a clusterskeleton having a continuous hole and a plurality of stable ionic sitesprovided apart from each other in the continuous hole and a metal ionincluded in any one of the stable ionic sites and formed movable to theother hollow stable ionic site, the molecular polarization being shownin a state in which the metal ion is included in any one of the stableionic sites, the molecular polarization being changed by movement of themetal ion to the other hollow stable ionic site by application of anelectric field, the method comprising at least: a step of specifying anyof a volatile memory, a storage class memory, and a non-volatile memoryas a use of the molecular memory; a step of determining a recordkeepingtime of the molecular memory based on the specified use; and a step ofselecting the metal ion based on the determined recordkeeping time suchthat the selected metal ion is included in the cluster skeleton.
 13. Themolecular memory manufacturing method of claim 12, further comprising:an oxidization-reduction step of changing the recordkeeping time bychanging an electronic state of the single-molecule electret byoxidization reaction or reduction reaction.